Due to the unreliability of traditional methods, it was necessary to investigate more dependable identification methods that can be read directly or by electronic means. A two-year study to determine the stability of and tissue reaction to a microchip glass-sealed device implanted in subcutaneous tissue of mice was conducted. Seventy B6C3F1 mice/sex were anesthetized and implanted with the microchip. The devices were read by an electronic detector and palpated at periodic intervals. Ten mice/sex were necropsied at 3 months and at 15 months with the remaining animals necropsied at 24 months. Of the 140 devices implanted, 3 were lost and 4 failed during the 24-month study. Devices were palpable and appeared to be fixed at one location with no obvious swelling due to inflammation or palpable masses around the implants for 24 months. At the 3, 15, and 24 month necropsies, implants were encapsulated by connective tissue. Light microscopic evaluation indicated that the capsule around the implants was thin and composed of fibrocytes and mature collagen fibers, with minimal to mild inflammation and occasional granulomatous reaction. Neoplastic changes were not observed in the tissue around the glass-sealed devices with polypropylene cap for up to 24 months.